Sensitivity Analysis of the High-Frequency-Link MMC to DC Link Voltage Ripples in a Back-to-Back Connected MMC-Based Power Electronic Transformer

Integrating renewables and storage to the medium voltage grid (5–34.5 kV), allows for high power export while minimizing conduction losses. Recent converter topologies involve cascading low-voltage (LV) submodules (SMs) to meet grid voltage levels, enabling power extraction from LV storage units and PV arrays. This article focuses on studying such a topology with a common controllable high voltage (HV) dc link for the grid side MMC (GS-MMC) and the high-frequency link MMC (HF-MMC). The storage units and renewable sources like wind and solar provide power through the LV side of a high-frequency (HF) step-up transformer, which is processed by the HF-MMC and GS-MMC, before reaching the grid. In this configuration, GS-MMC injects voltage ripples into the common HV dc link due to the employed pulse width modulation (PWM) scheme for switching its SMs. This injected ripple interacts with the switching modulation scheme of the HF-MMC SMs, resulting in voltage oscillations in the HF-MMC capacitors and current flow through its phase legs. The article provides a mathematical basis that explains the sensitivity of the HF-MMC response to different frequencies of voltage ripple. It also offers theoretical support for the observed resonant peaks in the frequency response of the HF-MMC phase legs, the mechanism behind the unique resonance and closed-form expressions for these resonant frequencies. An approximate equivalent circuit model for the HF-MMC phase legs is provided towards this end. The analysis is validated through MATLAB/Simulink simulations and OPAL-RT based Hardware-In-Loop real-time simulations, as well as experimental results obtained using a scaled-down laboratory prototype.